Grala bettered Tyler Reddick (129.449 mph) by .113 seconds.
The two teenagers making their debut landed in the mid-teens. Riley Herbst (128.200) was 15th on the chart; Justin Hayley (127.997) was 18th.
Cole Custer had the quickest 10-lap average with a speed of 127.576 mph.
Christopher Bell paced the first practice session for the Iowa 250 with a speed of 129.790 mph.
Daniel Hemric was second on the speed chart at 129.656 mph.
With time running off the clock, Shane Lee (129.603) fired off pit road and landed third.
Elliott Sadler (129.178) and Cole Custer (128.628) rounded out the top five.
Making his Xfinity debut, Truck series regular Justin Haley posted the sixth-fastest lap of 128.508 mph.
Riley Herbst posted a lap of 127.503 mph in his debut and landed 16th on the fastest lap chart.
Austin Cindric had the quickest 10-lap average of 126.074 among six drivers who posted at least 10 consecutive laps.